In a conventional refrigeration system, the capacity of an air cooled condenser is proportional to the temperature difference between the condensing temperature of the refrigerant and the ambient air temperature entering the condenser. The condenser is usually designed to operate efficiently at a temperature difference which is suitable for summer conditions. In winter conditions, the capacity of the condenser increases substantially because of the reduction in the ambient air temperature which enters the condenser. When the capacity of the condenser increases, the system-head pressure and the liquid-line pressure decrease, the liquid refrigerant in the liquid supply line which feeds the expansion valve flashes to a gaseous state, and consequently the amount of liquid refrigerant which is available to the evaporator is reduced. Because of these problems, a head-pressure control mechanism is required in colder ambient conditions to elevate the head pressure, thereby, increasing the efficiency of the system.
Many methods of controlling the head-pressure have been used. One such method regulates the amount of the ambient air which passes through the condenser by either cycling the fans or by controlling the speed of the fan motors. Alternatively, dampers have been used to limit the airflow through the condenser. Backflooding of the liquid refrigerant into the condenser, which limits the condensing surface, also achieves head-pressure control. Many such control systems have been proposed or are in use, such as those systems described in: U.S. Pat. Nos. 2,934,911; 2,986,899; 2,954,681; 2,963,877; 3,905,202; 4,068,494; 4,373,348, and 4,457,138.
Backflood of the condenser results in the sub-cooling of the liquid refrigerant which is in the condenser. By sub-cooling the liquid refrigerant, there is less need to use some of the latent heat of evaporation to cool the liquid refrigerant from the condensing temperature to the temperature ion takes place. This increases the efficiency at which evaporation takes place. This increases the efficiency of the system. The value of the sub-cooled liquid is usually lost, however, because the sub-cooled liquid is mixed with the discharge gas at the head-pressure control valve prior to entering the receiver or it is mixed with the discharge gas being diverted to the receiver.
Some systems bypass the sub-cooled liquid around the receiver such as described in Pat. Nos. 3,905,202, 4,430,866, 4,457,138, and 4,522,037. These systems use a "surge" receiver whereby the condenser drain line makes a three way connection with the bottom of the receiver and the liquid line supplying the evaporators. In warmer weather these systems retain the problem of controlling the amount of uncondensed gas which passes to the expansion valve from the condenser. Dealing with this problem, a subcooler in the liquid supply line to the evaporators can be used to condense the flash gas but this requires additional compressor capacity.
Expansion valves are designed to operate with only liquid entering at their inlet ports. The entrance of uncondensed gas into the expansion valve reduces the efficiency of the expansion valve so that an inadequate supply of liquid refrigerant is sent to the evaporator, and the efficiency of the system is lowered.
Dealing with this problem, Taft et al., U.S. Pat. No. 3,905,202, Willitts, U.S. Pat. No. 4,430,866, and Ares et al., U.S. Pat. No. 4,522,037 suggest that an evaporative sub-cooler should be used in the liquid supply line which leads to the expansion valve to condense any flash gas that may occur. In effect, the evaporative sub-cooler can act as an additional condenser. Such systems require greater work from the compressor. Vana, U.S. Pat. No. 4,328,682 teaches that a solenoid valve, which is controlled by a sensing device that detects flashing in the liquid line, should be used to divert discharge gas to the top of the receiver. Bowman, U.S. Pat. No. 4,457,128 discloses a inlet pressure regulating valve that discharges into the bottom of the receiver which in warm weather conditions causes heating of liquid in the receiver. A temperature controlled solenoid valve that bypasses the receiver is also shown that connects up stream from the inlet pressure regulating valve which can interfere with backflooding of the condenser. My prior patent, O'Neal, U.S. Pat. No. 4,566,288, teaches that a liquid level sensor should be placed in a chamber at the outlet of the condenser to detect the passage of uncondensed gas and activate a solid state circuit to close a solenoid valve in the bypass line and prevent the flow of uncondensed gas to the liquid line. This design has worked very well, however, in some cases the cost of purchasing and installing the solid state circuitry is economically prohibitive. A simpler method of controlling the head pressure and preventing the flashing of gas into the expansion valve is described in my copending U.S. patent application No. 020,376, filed Mar. 2, 1987, now U.S. Pat. No. 4,735,059, issued Apr. 5, 1988, whereby the static pressure in a drop leg out of the receiver and a sub receiver prevents flash gas from entering the liquid line to the evaporators.